1. Field of the Invention
The present invention relates to a solid-state relay (SSR) for protecting a switching element from a lightning surge, and, more specifically, concerns a solid-state relay (SSR) which, in the event of a lightning surge during an off-state of the switching element, protects the switching element by forcefully turning the switching element on.
2. Description of the Background Art
With respect to conventional solid-state relays (SSR), the following arrangement has been proposed: a surge absorbing element (for example, a surge absorber) is parallel-connected to the load-connecting side of a switching element so that the surge voltage of a multiplexed lightning surge from the load side is limited to a value lower than the maximum rated voltage of the switching element so as to protect the switching element.
FIG. 4 shows a block diagram that indicates one example of an essential portion of a conventional solid-state relay. In FIG. 4, a solid-state relay (SSR) 50 is provided with an input circuit 2, a photo-coupler 3, a resistor R1, a thyristor 4 and a surge absorber 5, and in this arrangement, a DC power supply VDC (or an AC power supply Vac) is applied between input terminals I1 and I2, and a load LD is connected to output terminals O1 and O2 with an AC power supply VAC being applied to the load LD. Here, the load LD is constituted by a resistor, a lamp, a valve, an electromagnetic clutch, etc.
The input circuit 2 is constituted by control circuits such as sequencers, and a control signal for controlling on-off states of the thyristor 4 serving as a switching element is supplied to a light-emitting diode of the photo-coupler 3 so as to allow an AC current to flow through the load LD or to stop it.
The photo-coupler 3 is formed by a photo-thyristor that is bi-directionally connected to the light-emitting diode, and when a control signal (for example, a DC current) is supplied from the input circuit 2, the light-emitting diode emits light so that, upon receipt of the light, the photo-thyristor is turned on with an AC current flowing through the resistor R1 and a resistor R2 of the thyristor 4.
The thyristor 4 is formed by a triac that is a bi-directional AC switch; thus, when the voltage drop of the resistor R2 (gate voltage) has reached a predetermined value, the triac is turned on bi-directionally, and in contrast, when the voltage drop of the resistor R2 (gate voltage) goes below a holding voltage, the triac is turned off to form a switching element.
When an AC current flows through the resistor R2 to cause a predetermined voltage drop, the thyristor 4 is turned on so that the output terminals O1 and O2 are short-circuited by a low impedance of the thyristor 4 to allow the AC current to flow through the load LD, thereby driving the load LD.
When the control signal (for example, a DC current) from the input circuit 2 is stopped, the photo-coupler 3 is turned off so that the thyristor 4 is also turned off to open the output terminals O1 and O2; thus, the AC current is blocked and the driving operation of the load LD is stopped.
Here, a bidirectional photo-thyristor is used as the photo-coupler 3 and a bi-directional triac is used as the thyristor 4 so that it is possible to switch an AC current having its polarity changed every half cycles. Moreover, the application of the photo-coupler 3 insulates the DC current on the input circuit 2 side from the AC current on the thyristor 4 side so as to separate these from each other.
The surge absorber 5 is parallel-connected to the thyristor 4, and placed on the output terminals O1 and O2 side; thus, in the case when a lightning surge that is induced by a transmission line and multiplexed on an AC power supply VAC is generated in the output terminals O1 and O2 through the load LD, this is determined by a surge resistant amount, and limited (surge-absorbed) to a surge voltage lower than the maximum rated voltage of the thyristor 4 so as to protect the thyristor 4 from the lightning surge.
FIG. 5 shows a block diagram indicating an essential portion of another example of a conventional solid-state relay. In FIG. 5, a solid-state relay (SSR) 60 is provided with an input circuit 2, a PD (photodiode) array coupler 9, a resistor R3, an MOS-type FET (electric field-effect transistor) 10 having n-channels and a surge absorber 5, and in this arrangement, a DC power supply VDC (or an AC power supply VAC) is applied between input terminals I1 and I2, and a load LD is connected to output terminals O1 and O2 with an AC power supply VAC being applied to the load LD.
The input circuit 2 is constituted by control circuits such as sequencers, and a control signal for controlling on-off states of the series-connected FET 10 (Q1 and Q2) serving as a switching element is supplied to a light-emitting diode of the PD (photodiode) array coupler 9 so as to allow an AC current to flow through the load LD or to stop it.
The PD (photodiode) array coupler 9 is constituted by n-number of photodiodes that are series-connected to the light-emitting diode, and when a control signal (for example, a DC current) is supplied from the input circuit 2, the light-emitting diode emits light, and upon receipt of the light, the n-number of photodiodes generate a predetermined voltage n×VD (VD represents a forward-direction voltage of the diode: approximately 0.6 V) so that a current is allowed to flow the resistor R3 with a bias voltage VGS being applied between the gate G and source S of the FET 10 (Q1 and Q2).
The FET 10, which is constituted by the FET Q1 and FET Q2 that are MOS-type FETs having n-number of channels, and series-connected to each other, is turned on by the bias voltage VGS (=n×VD) supplied from the n-number of photodiodes of PD (photodiode) array coupler 9 so that the output terminals O1 and O2 are short-circuited by a low impedance at the time of the on-states of the FET Q1 and FET Q2; thus, an AC current is allowed to flow through the load LD to drive the load LD.
When the control signal (for example, a DC current) from the input circuit 2 is stopped, the FET 10 is set to the off-state without the bias voltage VGS (=n×VD) supplied from the n-number of photodiodes of the PD (photodiode) array coupler 9 so that the connection between the output terminals O1 and O2 are opened to block the AC current, thereby stopping the driving operation of the load LD. In this manner, the FET 10 forms a bi-directional switching element for the AC power supply VAC.
Here, the n-number of photodiodes are used in the PD (photodiode) array coupler 9 and the FET 10 is used as the switching element so that it is possible to switch an AC current having its polarity changed every half cycles. Moreover, the application of the PD (photodiode) array coupler 9 makes it possible to insulate the DC current on the input circuit 2 side from the AC current on the thyristor 4 side so as to separate these from each other.
The surge absorber 5 is parallel-connected to the FET 10 and placed on the output terminals O1 and O2 side, and in the case when a lightning surge that is induced by a transmission line and multiplexed on an AC power supply VAC is generated in the output terminals O1 and O2 through the load LD, is determined by a surge resistant amount, and limited (surge-absorbed) to a surge voltage lower than the maximum rated voltage of the FET 10 so as to protect the FET 10 from the lightning surge.
With respect to the conventional solid-state relays (SSR) 50, 60, in the case when the maximum rated voltage of the switching element constituted by the thyristor 4, the FET (electric field effect transistor) 10 or the like is set to a value higher than the surge resistant amount (surge clamp voltage) of the surge absorber 5 with the lighting surge (surge voltage) to be multiplexed on the AC power supply VAC being set to a comparatively low level, it is possible to protect the switching element; however, in the case when the lightning surge (surge voltage) to be multiplexed is great, the surge absorber 5 fails to absorb this, with the result that the surge voltage to be applied to the switching element exceeds the maximum rated voltage of the switching element, and tends to damage the switching element.
The damage to the switching element to be caused by the lightning surge can be prevented by adopting a surge absorber 5 having a greater surge resistant amount (having a low surge cramp voltage with a great absorbing capacity) or a switching element (thyristor 4, FET 10) having a greater maximum rated voltage; however, large-size surge absorber and switching element are required, consequently resulting in high costs.
Here, it has been known that the influences of lightning surge are very small (causes no damage) when the switching element is in the on-state, and very serious when the switching element is in the off-state (causes damages).